1502 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



it did not occur in ethanol. In a mixture containing 5 X 10 ~® mole/liter 

 chlorophyll and up to 5 X 10^^ mole/liter ascorbic acid, illuminated by a 

 500-watt lamp through a red filter (and a layer of water), the chlorophyll 

 fluorescence disappeared in 1-2 min. Phenylhydrazine also could be used 

 as a reductant (instead of ascorbic acid). Some reversible reduction could 

 be observed in alcoholic solution if a little pyridine or ammonia were added 



1.000- 



N 



o 



0.500 



450 



500 550 



600 



650 



700 



m^ 



Fig. 35.3. Reversible reaction of chlorophyll a with ascorbic acid in pyridine in 

 light (after Krasnovsky 1948): (i) spectrum before reaction; {2) spectrum after 

 reaction; (5) appro.ximate spectrum of the unstable reaction product (6 min. after 

 darkening). 



to alcohol. No reaction was observed (in pyridine) with pyruvic, oxalic, 

 malic or citric acids, ethanol, or hydroquinone as reductants. 



The dark back reaction required 2-3 hours. After complete recovery, 

 the spectrum was very similar to that of original chlorophyll, especially 

 when chlorophyll a was used. The chlorophyll h band at 470 m/x in an 

 (a + h) mixture was weakened after the cycle, indicating, in Krasnovsky's 

 first surmise, irreversible reduction of the carbonyl group (which distin- 

 guishes chlorophyll h from chlorophyll a). 



Figure 35.3 shows the spectrum of chlorophyll a before the reversible 

 reaction with ascorbic acid in air-free pyridine (curve 1), and after this re- 



